It is known to use aromatic polyamide fibers as fibers for reinforcement of rubbers.
Aromatic polyamide fibers are featured in that they have high strength and high modulus, are excellent in heat resistance and dimensional stability, and do not rust because of organic fibers. Therefore, they are utilized as fibers for reinforcement of rubbers. However, aromatic polyamide fibers have the defect of insufficient adhesion to rubbers. That is, if adhesion between fibers and rubbers is insufficient when fibers are used for reinforcement of rubbers, the effect as reinforcing materials is not sufficiently exhibited. In this respect, aromatic polyamide fibers are inferior to fibers such as nylon, rayon, and vinylon.
Attempt has been made to enhance adhesion of such aromatic polyamide fibers to rubbers. For example, JP-A-59-94640 discloses a process which comprises treating aromatic polyamide fibers with water-soluble epoxy compounds and then treating the fibers with so-called RFL solution comprising resorcin-formaldehyde resins and rubber latices. Furthermore, JP-A-56-2156 discloses use of RFL solution containing reactive materials such as ethylene urea compound.
However, the conventional processes have problems that relatively high adhesive strength is obtained at an initial stage, Out adhesive strength markedly reduces when a reinforced rubber composition is used in contact with water, namely, a problem of poor water resistance.
Under the circumstances, a principal object of the present invention is to provide a process for producing aromatic polyamide fibers which not only nave high adhesion at the initial stage, but also are little reduction in adhesive strength even after they are dipped in water for a long period, namely, excellent in water resistance.